The properties of a bag filter are largely determined by the nature of the filter medium used in the construction of the bag filter and the manner in which that filter medium is formed into the bag filter. Thus, many of the deficiencies of a particular bag filter are the result of deficiencies inherent in the filter medium used in the bag filter and/or the bag filter construction technique (particularly as regards seam sealing).
Bag filters may be prepared from fibrous nonwoven webs. A nonuniform fibrous nonwoven web utilized in the construction of a bag filter can result in a bag filter with a nonuniform pressure drop, poor tear resistance, and/or unsatisfactory filtration efficiency. A nonuniform fibrous nonwoven web may result in the need to use multiple layers to overcome some of these deficiencies.
Various techniques of preparing melt-blown fibrous nonwoven webs have been disclosed. For example, in U.S. Pat. No. 3,825,380, the Exxon melt-blowing system is described; however, this system results in fibrous nonwoven webs with undesirable fiber roping and twinning. Both twinning and roping cause the fibrous nonwoven web to have a relatively high pressure drop and low filtration efficiency. The presence of shot, i.e., small pellets of unfiberized resin interspersed in the web, can also induce irregular pore size and be a problem with such fibrous nonwoven webs. U.S. Pat. No. 4,021,281 describes a method in which attenuated fibers are produced by a system which bears similarities to the Exxon system. The resulting fibrous nonwoven web contains many of the same deficiencies present in the fibrous nonwoven web prepared using the Exxon system.
The fibrous nonwoven webs are generally in the forms of sheets which are formed into a bag filter by folding the sheet into a tubular form, sewing together the adjoining portions and one end of the tube, typically turning the resulting bag inside-out, and then heat-sealing the seams with thermoplastic tape. The bag filters thus produced have two seams, namely a side seam and an end seam. For instance, U.S. Pat. No. 5,156,661 discloses a bag filter comprising at least one sheet of a filter medium, preferably a polypropylene high dirt capacity filter medium, formed into a bag configuration with an opening, an inside surface, an outside surface, and at least one seam formed at adjoining filter medium portions which have been sewn together, wherein the seams are sealed with a thermoplastic tape that has been heat-sealed to the adjoining filter medium portions.
Similarly, U.S. Pat. No. 5,205,938 discloses a bag filter comprising at least one sheet of a filter, preferably a polyester high dirt capacity filter medium having a graded pore structure, formed into a bag configuration with an opening, an inside surface, an outside surface, and at least one seam formed at adjoining filter medium portions which have been sewn together. The seams are optionally sealed with a thermoplastic tape that has been heat-sealed to the adjoining filter medium portions.
Bag filters produced by sewing and heat-sealing have certain deficiencies. Since the seams have been punctured by the sewing needle and are held together by the thread, the structural integrity of the seam is weakened and hence that of the bag filter itself. Moreover, sewn seams can provide fluid leakage pathways during use. In addition, the method of producing such bag filters is time-consuming and costly because of the number of steps involved when the seams are sewn and heat-sealed with a thermoplastic tape.
Some bag filters are formed by merely thermally sealing the seams, i.e., bonding the fibrous web to itself without any sewing. Although such a sealing technique avoids the need to puncture the fibrous nonwoven web with a sewing needle, the thermal sealing technique also results in two seams (namely, a side seam and an end seam) and suffers from other deficiencies. In particular, the thermal sealing technique requires the partial melting of the fibrous nonwoven web, thereby adversely affecting the structural integrity and filtering characteristics of the bag filter.
Bag filters free of a side seam have been disclosed. For example, U.S. Pat. No. 4,021,281 discloses the production of nonwoven tubular webs from which bag filters may be produced. Although the bag filters are free of side seams, one end of the tube is still subject to the usual closing steps, namely, sewing and heat-sealing with a thermoplastic tape or thermally sealing the fibrous nonwoven web to itself. Thus, there is created an end seam which has the same deficiencies as the side seam, namely, the weakened structural integrity of the bag filter, possible fluid leakage pathways, and the time-consuming steps involved in the closing.
Many bag filters have a collar attached to the open end so that the filter may be suitably positioned in the filter holder during use. The collar is generally stitched and heat-sealed to the opening of the bag filter, or merely thermally sealed directly to the bag filter at its opening, thereby creating a collar seam. For the same reasons indicated above with respect to the side seam and end seam, a collar seam can be similarly problematic.
Accordingly, there remains a need for improved bag filters. In particular, there is a need for bag filters which avoid some or all of the problems associated with nonuniform filter media and sewn or thermally sealed seams. The present invention seeks to provide such a bag filter. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.